Activity in the optimization of performance for the erbium doped fiber amplifiers (EDFA) has been motivated by their potential applications in communication networks. In network topologies with large fanouts as for example star topologies, the erbium doped fiber amplifier is utilized to overcome splitting losses. As an in-line amplifier, these devices have application in long distance communications where erbium doped fiber amplifiers are expected to increase considerably the repeater spacing. A typical configuration for the erbium doped fiber amplifier consists of a pump laser operating at a wavelength of 1480 nm or 980 nm which is combined with the signal of wavelength 1550 nm into the erbium doped fiber using either a dichroic beam splitter or a fiber wavelength division multiplexer (WDM). The amplified signal at 1550 nm and a portion of the pump radiation appear at the output where they are separated using again either a dichroic beam splitter or fiber WDM.
To maximize the pump intensity in the erbium doped fiber amplifier, bi-directional pumping is commonly utilized. This involves pumping with two lasers, one from each end of the amplifier. For optimal performance a high numerical aperture (N.A.) fiber reduces the mode field diameter of the pump and signal thereby increasing its intensity and therefore gain.
The most expensive component in the erbium doped fiber amplifier is the pump laser primarily due to the high power requirement. Of the two most common types of pump lasers, 1480 and 980 nm, the later is more efficient and also results in lower noise in the amplifier, making it the wavelength of choice from the perspective of performance. However, the technology of fabricating these 980 nm InGaAs strained layer lasers is relatively new and is still developing. An important issue is the reliability of these pump lasers. These lasers have a typical lasing threshold of 30 mA and a differential quantum efficiency of 0.6-0.8 mW/mA. It is also becoming clear that the lifetime of these lasers is significantly degraded when operated at currents above 150 mA. This places an upper limit of approximately 95 mW on laser power which is further reduced by factors such as laser-fiber (pigtail) coupling and losses associated with the 980/1550 WDM couplers. Taking these losses into account, about 40 mW of pump power can be coupled into the erbium doped fiber. Power amplifiers with an output signal power of approximately 15 dBm require approximately 100 mW of 980 nm pump power in the active fiber. Given the constraints described above, it is not practical to achieve this level of pump power even using bi-directional pumping using two pump lasers. Taking these factors into account it becomes desirous to increase the pump power without compromising the reliability of pump lasers in the erbium doped fiber amplifier.